Santorum's attacks could hurt Romney significantly, not just by depressing turnout but by reinforcing the deep divide in today's Republican Party between the pro-Romney establishment and the anti-Romney rest of the party.
Mitt Romney's rivals know that attacking him will only weaken him for the general election, assuming he wins the nomination. But at least a couple of his rivals are actually trying to win the nomination.
Barbour's statement is significant for two reasons: First, it sounds like the Mississippi governor is indeed running for the GOP presidential nomination. Second, it suggests his state has changed considerably since the 1960s.